Porson used to go to the Cyder-cellar: he said that a stranger, whom he met there, used to discourse most learnedly about the οἶνος κρίθινος, or whiskey of the ancients, as this stranger asserted it to be; he surprised Porson with the accuracy, variety, and extent of his information. He was never found out, notwithstanding all Porson’s enquiries. A fortnight afterwards Porson was asked about the existence of the Devil: "Sir," said he, "I doubted his existence till I saw him seated in that chair a fortnight ago."

Thanks to Andrew Rickard for introducing me to Barker's book, where I also find (vol. II, p. 23) this:

The following anecdote is from Mr. Cogan. Poison called on a friend, who was reading Thucydides, and wished to consult him on the meaning of a word. Porson, hearing the word, repeated the passage. His friend asked how he knew it was that passage. "Because," said P., "the word occurs only twice in Thucydides, once on the right hand, and once on the left. I observed on which side you looked, and therefore knew the passage to which you referred."